In a common steam power plant three cooling circuits are used to perform various cooling operations.
A first cooling circuit is often termed “circulating water piping and culvert system” or main cooling circuit, abbreviated “PAB”. It provides the highest cooling power in the power plant. It comprises a condenser and a first pipe system that is split into a hot part and a cold part. The cold part connects a cold-fluid outlet of a cooling tower with an inlet of the condenser. The hot part connects a hot-fluid inlet of the cooling tower with an outlet of the condenser. The cooling tower is used to cool a first cooling fluid. The PAB has at least one first pump, which is often termed circulating water pump or main pump. The first pump is located inside of the cooling tower. When switched on, the first pump pumps a first cooling fluid (water) from the cooling tower through the cold part of the first pipe system, through the condenser where it heats up, through the hot part of the first pipe system and back to the cooling tower. In the condenser the first fluid cools down the condenser. The cooled condenser cools down the hot steam to condense the steam. The hot steam is fed into the condenser after departing from a steam turbine. The steam turbine is driven by the steam and used to drive a generator that generates electricity.
A second cooling circuit is often termed “service water piping and culvert system” or auxiliary cooling system, abbreviated “PCB”. It comprises a second pipe system and a heat exchanger. The heat exchanger comprises an inlet and an outlet. Also the second pipe system is split into a cold part and a hot part. The cold part connects the cold-fluid outlet of the cooling tower with the inlet of the heat exchanger. The hot part connects the hot-fluid inlet of the cooling tower with the outlet of the heat exchanger. At its hot part the second pipe system is partly realized by a section of the first pipe system that is connected with the cooling tower. This part is termed “first common pipe section”. Also at its cold part the second pipe system is partly realized by a section of the first pipe system that is connected with the cooling tower. This part is termed “second common pipe system”.
The PCB uses the first cooling fluid to cool down the heat exchanger. Only during operation of the first pump the first cooling fluid flows from the cooling tower through the cold part of the second pipe system, the heat exchanger where it heats up and the hot part of the second pipe system back into the cooling tower.
Sometimes the second cooling circuit is equipped with a small booster pump located in its cold part but not in the second common pipe section. The booster pump boosts the flow of the first fluid after it branches of from the second common pipe section. The booster pump is necessary if a plate heat exchanger is used. In operation the booster pump increase the pressure in the plate heat exchanger if the pressure drop over the PCB is higher than over the PAB. This ensures a sufficient high flow rate of the first cooling fluid that flows through the plate heat exchanger, which in turn avoids damages of the heat exchanger. The operation of the booster pump is always synchronized with the operation of the first pump.
The third cooling circuit is often termed “closed cooling water system” or component cooling system, abbreviated “PGB”. It is a closed cooling circuit that comprises a third pipe system and a number of component coolers that are thermally coupled with components to be cooled. Also the heat exchanger is part of the PGB. The third pipe system connects the heat exchanger with the component coolers. The component coolers are commonly known and a non-compulsory list of such component coolers may comprise: condensate pump coolers, coolers for a HTF-system (including pumps etc.), evacuation pumps coolers (e.g. for the condenser), ST lube oil coolers, generator coolers, feed water pump coolers, sampling coolers, and so on. The PGB also shows a number of closed cooling water pumps to pump a second cooling fluid (water). In the PGB the second cooling fluid is circulated between the heat exchanger and component coolers. In the heat exchanger the first cooling fluid is thermally coupled with the second cooling fluid but physically kept separate from the second cooling fluid. Heat is transferred from the second cooling fluid into the first cooling fluid.
A problem of the known steam power plant and the known method is that a proper cooling of the components can only be achieved by the aid of the first pump being in operation. The first pump is a main pump and as such without an operation of the main pump the entire cooling system is out of service. This is of particular disadvantage in case of solar steam power plants that do not have heat storage means for power generation during the night. In general, the power plant does not deliver power during its standby-mode, e.g. night operation. Typically, the solar steam power plant is driven into the standby mode on a daily basis during the night hours. Sometimes, e.g. during winter season, the standby mode is selected even multiple times a day. In contrast to its power-mode operation (power generation operation) it consumes power during the standby-mode because some components must remain in operation in order to rapidly re-start the power-mode in the morning. In addition, although no steam for generating power is produced by solar radiation, a so termed “sealing steam” is generated. In practice, saturated auxiliary steam is produced which is than overheated and becomes sealing steam. The sealing steam is fed into the turbine separately from steam used to produce electricity. From the turbine the sealing steam is at least partly guided into the condenser. Hence, in order to prevent the components and the condenser from damages due to overheating there must be a cooling operation during the standby-mode. The cooling of the components and the condenser requires the main pump to be active. In the cooling system the main pump is one of the largest power consumers and as a consequence the entire power plant shows a relatively low efficiency. A control unit of the plant keeps the main pump switched on during standby-mode.